To improve a performance of a photovoltaic device such as a solar cell, it is important that how efficiently solar light can be taken into the photovoltaic device. Therefore, conventionally, a texture structure intentionally having fine asperities of a size from a few dozens nanometers to a few dozens micrometers formed on a surface of a light incident side has been manufactured. According to this conventional texture structure, light once reflected from the surface is input to the surface again to take more solar light into the photovoltaic device, thereby increasing generated currents and improving the photoelectric conversion efficiency.
As a method of forming a texture structure on a solar cell substrate, when the substrate is a monocrystalline silicon (Si) substrate, there has been widely used an anisotropic etching process that utilizes a crystal orientation of an alkali aqueous solution, such as sodium hydroxide and potassium hydroxide, the etching rate of which has a crystal orientation dependency (see, for example, Patent Document 1). For example, when the anisotropic etching process is performed on a substrate surface having a (100) surface orientation on the surface, a texture of a pyramid shape having a (111) surface exposed is formed.
However, in the case of a polycrystalline silicon substrate, according to a method of performing an anisotropic etching process by using an alkali aqueous solution, crystal surface orientations of crystal particles constituting a substrate surface are not aligned. Further, etching rates of an anisotropic etching process itself using an alkali aqueous solution are greatly different depending on a crystal surface. Therefore, a texture structure can be manufactured only partially. Under these circumstances, in the case of the polycrystalline silicon substrate, there is a limit to a reduction of a reflection rate. For example, the reflection rate at the wavelength of 628 nanometers is about 36% in the case of silicon of which surface is mirror polished, and the reflection rate is about 15% in the case of a monocrystalline silicon substrate of a (100) surface which is wet etched. On the other hand, the reflection rate is about 27% to 30% when a polycrystalline silicon substrate is wet etched.
As a method of forming a texture structure on the entire surface regardless of its crystal surface orientation, a mixed acid etching method using an etching mask has been proposed (see, for example, Patent Document 2). As for a manufacturing method of an etching mask, there can be used a method according to lithography, which is used in a semiconductor process, or a method of mixing fine particles of low etching resistance into a liquid solution of an etching resistance material and coating the mixture onto a substrate surface.
In this way, for example, a low-concentration N-type diffusion layer, in which an N-type impurity is diffused in a low-concentration, is formed on the entire surface of a P-type silicon substrate at a formation side of a texture structure, while a high-concentration N-type diffusion layer, in which an N-type impurity is diffused in a high-concentration, is formed at a light-incident-side electrode formation portion. Further, a grid electrode made of a metal such as silver and arranged in a comb shape, and a bus electrode made of a metal such as silver that collects currents from the grid electrode are formed at the light-incident-side electrode formation portion. A back-surface electrode made of a metal such as aluminum and silver is then formed on a back surface of a silicon substrate. With this arrangement, a photovoltaic device can be obtained.
Patent Document 1: Japanese Patent Application Laid-open No. H10-70296
Patent Document 2: Japanese Patent Application Laid-open No. 2003-309276